【作者】 丁汉清；

【导师】 杨家玮；

【作者基本信息】 西安电子科技大学 ， 军事通信学， 2011， 博士

【摘要】 移动通信技术的发展为人们提供了越来越强大.便捷的通信手段,正深刻地改变着人们的生产与生活方式?但是,随着全球范围内移动用户数的快速攀升?互联网业务的迅猛增长以及便携计算机设备的广泛使用,通信系统对无线频谱资源的需求也在不断增加?在自然的频谱资源有限的情况下,现有的静态频谱分配方案已显然不能满足高速无线通信业务快速增长的需求,因此,需要开发新的技术,为新的业务提供更多可用的频谱?认知无线电能够通过择机利用主用户空闲的无线频谱资源,被认为是解决当前无线频谱短缺问题的有效技术?同时,认知无线电技术是一项复杂的技术,其研究内容包括频谱感知?频谱共享和频谱管理等诸多方面?本文主要对认知无线电频谱感知技术进行了研究,主要研究内容和取得的创新性成果如下:1.提出了一种自适应分步合并协作频谱感知算法?传统的合作频谱感知方法中,等增益合并协作频谱感知可以获得接近最优软合并的检测性能,是认知系统中常用的频谱感知方法,但是,当参与合作的认知用户较多时,通过控制信道向融合中心传送本地检测统计量会占用大量的信道资源,这对带宽受限的认知无线电系统来说是不利的?为了缓解这个问题,在分析传统合作频谱感知性能的基础上,提出了一种自适应分步合并协作频谱感知算法,根据认知用户接收信噪比的变化,通过自动调整参与协作频谱感知的认知用户数,以减少认知无线电网络中的数据开销?推导了在瑞利衰落信道下采用此算法时,认知用户通过控制信道向融合中心发送的平均数据量的计算公式,理论推导和仿真结果表明,和等增益合并协作频谱感知算法相比,在保证频谱感知性能的前提下,该算法减少了认知用户通过控制信道向融合中心发送的平均数据量.2.提出了一种基于分群的自适应双门限合作频谱感知算法?认知无线电网络中,合作频谱感知的性能会受到诸多因素的制约,除了通知信道带宽受限对合作频谱感知性能的提高是一个挑战之外,通知信道的可靠性对合作频谱感知性能也有很大的影响,实际应用中,随着认知网络范围的扩大,同感知信道一样,通知信道也会受到多径衰落和阴影等因素的影响,频谱感知性能可能会因此严重下降?针对这些问题,提出了一种基于分群的自适应双门限合作频谱感知算法:认知用户被分成若干群,选择群内通知信道增益最大的认知用户为中心节点,采用自适应双门限机制负责群内认知用户进行合作频谱检测,并将检测结果报告给认知基站进行合并,以确定主用户信号是否存在?仿真结果表明:相对于传统的合作频谱感知,一方面,该算法能利用分集技术降低通知信道传输错误率,缓解通知信道衰落对合作频谱感知性能的影响,另一方面,通过自适应双门限机制,能进一步减少认知系统频谱感知过程中的平均数据开销,避免控制信道拥塞,降低感知延迟.3.提出了一种通过优化感知时间使认知用户平均信道效率最大的方法?频谱感知的目的是快速可靠地发现主用户的频谱空穴,因此,如何提高频谱感知效率,有效利用频谱空穴是频谱感知中的一个重要问题?以能量检测作为频谱感知的基本方法,以最大化认知用户平均信道效率为目标,分析了单用户频谱感知情况下,在认知用户的不同活动阶段,感知时长对认知用户平均信道效率的影响,提出了在不对主用户造成有害干扰的前提下,通过优化感知时间使认知用户平均信道效率最大的方法,并通过仿真对理论分析进行了验证.更多还原

【Abstract】 The development of mobile communication technology has provided a more powerful and convenient means of communication and it has profoundly changed people’s production and life. However, as the rapid rise of the number of worldwide mobile subscriber, the growth of Internet business and the wide use of portable computer equipment, the need for wireless spectrum is also increasing rapidly. It is leading to a shortage for spectrum resources. Given the limitations of the natural frequency spectrum, it becomes obvious that current static frequency allocation schemes can not accommodate the requirements of the increasing of the number of higher data rate devices. As a result, innovative techniques that can offer new ways of exploiting the available spectrum are needed. Cognitive radio arises to be a tempting solution to the spectral congestion problem by introducing opportunistic usage of the frequency bands that are not heavily occupied by licensed users. Cognitive radio offers a variety of research fields, including spectrum sensing, sharing and management. This dissertation deals with the problem of spectrum sensing in cognitive radio. The main achievements and results are listed as follows.1. An adaptive step-by-step combination cooperative spectrum sensing in cognitive radio network is proposed. Among the traditional cooperative spectrum sensing methods, equal gain combining cooperative spectrum sensing is commonly used in cognitive radio systems as it has the performance close to that of the optimal soft-combining spectrum sensing. However, when the number of cognitive users involved in cooperation becomes larger, it takes a lot of resources to send local test statistics to the fusion center through control channels. This is negative to the cognitive radio (CR) system whose bandwidth is limited. To alleviate this problem, based on the analysis of traditional cooperative spectrum sensing, an adaptive step-by-step combination cooperative spectrum sensing in cognitive radio network is proposed. With the change of the signal-to-noise ratio received by CR users, this algorithm can adjust automatically the number of CR users which participate in cooperative spectrum sensing in order to decrease the data overhead produced by spectrum sensing in the cognitive radio network. The formula for computing the average data magnitude from the CR users to the fusion center over Rayleigh channels is acquired. Theoretical analysis and simulation results show that the method reduces the average data overhead from the CR users to the fusion center under the same detection performance compared with the conventional equal gain combination cooperative spectrum sensing.2. An adaptive dual threshold cooperative spectrum sensing based on grouping is proposed. In cognitive radio networks, the performance of cooperative spectrum sensing will be subject to many factors. In addition to bandwidth requirement being a challenge to the performance improvement of cooperative spectrum sensing, the reliability of the reporting channels has the great impact on cooperative sensing performance. Actually, with the expansion of the scope of cognitive network, the reporting channels are susceptible to multipath fading and shadowing like sensing channels. Hence, the channel impairments must be considered in the reliability issue of control channel. The adverse effects on sensing performance due to the reporting channel fading can not be ignored. It may lead to serious decline of cooperative spectrum sensing. To solve these problems, an adaptive dual threshold cooperative spectrum sensing based on grouping is proposed. All CR users are divided into several groups. In each group, the one with the largest reporting channel gain is selected as the central node. After performing cooperative spectrum sensing with adaptive dual threshold, the central node forwards its sensing result to the cognitive radio base station (CRBS), and then the CRBS fuses the results from all groups to decide the presence or absence of the authorized users. Simulation results indicate that this method can lower the error rate of the reporting channel through selection diversity. By adopting the adaptive dual sensing mechanism, it also decreases the average data overhead. Consequently, the congestion on control channel can be alleviated and sensing delay can be reduced.3. A method is proposed that maximizing the average channel efficiency of cognitive radio user by optimizing the spectrum sensing time. The purpose of spectrum sensing is to find white space of licensed spectrum fast and reliably. Therefore, how to improve the efficiency of spectrum sensing is an important issue. With the energy detection as the underlying method of spectrum sensing and maximizing the average channel efficiency of CR user as our target, the effect of sensing durations in different stages on the average channel efficiency of CR user is analyzed in the context of spectrum sensing with a single user. The sensing durations that maximize the CR user’s average channel efficiency are derived while providing the primary users with their desired level of interference protection. The theoretic analysis is verified by the simulation results.更多还原